1. Field of the Invention
The present invention relates to an image processing apparatus, an image processing method, and a storage medium.
2. Description of the Related Art
Ophthalmic examinations are prevalently made for the purpose of earlier diagnoses of various diseases that come before lifestyle-related diseases and causes of blindness. An ophthalmic tomography apparatus such as an OCT (Optical Coherence Tomography) is expected to be helpful to give more adequate diagnoses of diseases since it allows to three-dimensionally observe the state of the interior of retina layers. In order to quantitatively measure thicknesses of layers, boundaries of respective retina layers are detected from a tomogram using a computer. For example, as shown in FIG. 10B, an inner limiting membrane B1, inner plexiform layer boundary B2, boundary B3 between photoreceptor inner and outer segments, and retinal pigment epithelium boundary B4 are detected, and retina thicknesses T1 and GCC (Ganglion Cell Complex) thicknesses T2 are measured.
Tomograms of an eye portion captured using the OCT include regions (cells, tissues, portions) which influence a visual function, and a doctor observes conditions of damages of corresponding cells, layer thicknesses, and positional relationships with a portion such as a fovea centralis F1 (FIG. 10A) upon predicting the restoration possibility and prognosis of the visual function. For example, FIG. 10B shows an enlarged view of a region Ro in FIG. 10A. Light which has reached a retina is converted into an electrical signal by an outer segment L1 of a photoreceptor cell C1, and is perceived by a visual cortex (not shown) of the cerebrum via a bipolar cell C2, a ganglion cell C3 and optic nerve (not shown) in turn. If the photoreceptor outer segment L1 is damaged, since it can no longer convert light into an electrical signal, the visual function lowers at the damaged portion. The photoreceptor cell C1 includes a petrosa and rod, and the petrosa controls the visual function in a bright place. As shown in FIG. 10C, since the petrosa is present at a higher density as it is closer to the fovea centralis F1, the influence of the photoreceptor outer segment damage on the visual function per unit area is greater as it is closer to the fovea centralis F1. Note that a visual axis is a line which connects an object to be gazed and the fovea centralis F1. Therefore, in order to estimate the influence of the photoreceptor outer segment damage on the visual function, a degree of damage (length of the outer segment) at the position of the photoreceptor outer segment damage, an existence range (area) of the damage region, and a distance of the damage region from the fovea centralis have to be taken into consideration.
Furthermore, since the photoreceptor cell C1 derives nutrition from choroidal vessels V1, as shown in FIG. 10D, if a lesion such as an amotio retinae RD exists between the photoreceptor outer segment L1 and a retinal pigment epithelium L2, nutrition cannot be supplied to the photoreceptor cell C1. Therefore, if the amotio retinae RD exists, the visual function is more likely to decrease in the future. If existence of the amotio retinae RD is protracted, the photoreceptor cell C1 becomes extinct, and the visual function can no longer be restored. Therefore, in order to estimate the influence on the visual function or to predict the prognosis of the visual function from the retina shape, not only the thicknesses of cells and layers are simply measured, but also the positional relationships with a portion such as the fovea centralis F1, and the presence/absence or existence period of a lesion such as the amotio retinae RD have to be taken into consideration. For the purpose of diagnosis support of a glaucoma and optic nerve disease, a technique of measuring the GCC thicknesses T2 related to a visual field as one of the visual functions, and displaying differences from GCC thicknesses T2 in a healthy eye as a map is disclosed in WO/2008/157406 (to be referred to as literature 1 hereinafter).
However, the technique described in literature 1 assumes an application to a glaucoma, and does not consider any positional relationship with the fovea centralis F1 and any influence of an exudative lesion on the visual function, which are to be considered in case of a macular disease. Also, the technique described in literature 1 presents only one or more layer thickness maps (parallelly) for predictions of the restoration possibility and prognosis of the visual function, and the restoration possibility and prognosis of the visual function have to be visually judged from the maps.
Also, a technique described in Japanese Patent Laid-Open No. 2009-34480 is premised on the presence of test results of a perimeter, and measures layer thicknesses of a portion, the anomaly of which was revealed by the visual field test to examine correspondence with visual function evaluation values by the visual field test. As is known, a change in retina shape appears prior to a change in visual function, and it is desirable to predict the prognosis based only on information obtained from ophthalmic tomograms in terms of early detection of a retinal disease.